The Heat is On

Running (and racing) in high temperatures means more than hydrating properly. Here's a summertime survival guide.

It was 9 o'clock in the morning and almost 90 degrees. Instead of sitting with my feet up in a breezy breakfast nook, I was exhausted on mile eight of the Hottest Half in Dallas. Raising my legs required every single recruitable muscle fiber. Moving my arms felt like swinging sandbags. Even breathing was tiring, each exhale laboriously propelled from the wheezing bellows inside me.

I didn't know it, but with 5.1 miles to go, my core temperature had risen to a 10th of a degree shy of possible heatstroke. I ignored my brain's urgent demand and kept running.

I hate the heat. It makes me grouchy, uncomfortable, and sluggish. As a runner, I live for cool spring mornings and the snap of crisp, autumnal air. But loathe heat or love it, you can't mess around with it.

"Heat can kill you," says William O. Roberts, M.D., medical director for the Medtronic Twin Cities Marathon in St. Paul, Minnesota. "That's why we have to be so careful with it, especially runners."

Running in sauna-like conditions can throw your internal equilibrium seriously out of whack. The body normally cools itself by moving blood—which is mostly water—to sweat glands in the skin, says Douglas Casa, Ph.D., A.T.C., COO of the University of Connecticut's Korey Stringer Institute. The glands create sweat droplets that carry heat to the surface of the skin, where it evaporates.

"The droplets of sweat are like little containers for the heat," Casa says. When you continue to run, your organs and working muscles compete for a limited blood supply, which compromises this cooling system. Humidity compounds the problem, by hindering the evaporation of sweat, making it harder to cool yourself. And runners performing intense exercise in hot weather tend to become dehydrated, says Casa. "With less water in the body, you have less blood plasma volume—the liquid portion of your blood—to serve all your needs."

Once your body temperature climbs to 104 degrees, you're in the heatstroke danger zone. Continued hard running at this temperature can overwhelm your cardiovascular system. Hit 105 degrees for 30 minutes or more and your body may start to cook from the inside out. The hyperthermia can weaken the heart, cause the kidneys and the liver to shut down, and cause cell damage. Exertional heatstroke has arrived.

A complicating factor for runners is that everyone reacts to heat differently. For example, average at-rest body temperature is 98.6 degrees; normal while running in the heat is 101 to just below 103. Some people who hit 103 will feel fatigued and light-headed, while others will feel fine. Same with 104 degrees. At this point, your body may be generating more heat than it can dissipate, and you may feel like death—or feel nothing at all. "The symptoms are the key," says Casa. "If you feel poorly—light-headed, nauseous, extremely fatigued, have cramps or a headache—those are big-time warning signs to back off the pace." Perhaps the most important symptom, he says, is when everything seems harder than it normally does. "That's a sign to ease up and slow down. Your body is now locked in an internal battle, trying desperately to keep itself cool while you are forcing it to keep working."

Ultimately, says Roberts, there's only one surefire way to prevent heat distress on a hot day: "Go lie in a hammock."